1. Field of the Invention
The present invention relates to a method capable of measuring the radiated power with a simple configuration and a small radio terminal in a short time with a high sensitivity, a coupler configured to measure the radiated power and an apparatus configured to measure the radiated power.
2. Description of the Related Art
With the arrival of the ubiquitous society, an explosive extension of ownership of and an increased demand for an ultra small radio terminal such as radio communication devices in terms of the radio frequency identification tag (RFID), the ultra wide band (UWB) and the body area network (BAN) are expected.
Many of these devices, unlike the conventional radio communication devices, have no test terminal due to the dimensional limitation or the economic reason. Any of these devices is required to be tested, therefore, by receiving the radio wave radiated by the particular device itself. Especially, the radiated power of the small radio terminals described above is strictly specified taking the effect thereof on the other communications and the human bodies into consideration. Thus, the measurement of the radiated power constitutes an important test item.
The radiated power includes the equivalent isotropically radiated power or the effective isotropically radiated power (EIRP) and the total radiated power (TRP) with the power radiated into the whole space. In measuring the radiated power, TRP has come to be used in more and more cases in view of the fact that EIRP requires a complicated measuring instrument and a long measurement time.
The well-known conventional TRP measurement methods so far used include the following:
(1) The spherical scanning method in which such points on a spherical surface that contain a device under test (DUT) are scanned with a probe, and by measuring and accumulating the radiated power at mesh points, the total radiated power is then determined.
(2) The method in which the radio wave radiated from a device under test (DUT) is agitated by the rotation of a metal blade thereby to generate a random field in a room covered with a metal, and the total radiated power (TRP) from the DUT is estimated based on a statistical technique.
(3) The method using a pyramidal space covered with a metal film and a device called a G-TEM cell (gigahertz transverse electromagnetic cell) which generates the TEM wave in a radio wave absorber.
(4) The method using an electromagnetic coupling device having a plurality of antennas, an isolator connected to each of the antennas, a phase regulator, a synthesizer configured to synthesize the signals of the array antennas, and the like to measure the radiated power from a device under test (DUT) arranged on the center line of the antenna array.
Incidentally, the spherical surface scanning method of (1) is disclosed in TECHNICAL REPORT OF IEICE AP2002-61 (2002-7), pp. 29-34, July 2002 “Simplified High Accuracy Measuring Method for Radio Equipment Using Integral Antennas, Radiated RF Power Measurement Using a Spherical Positioner (Part 1)”, Tomoyuki NOJIMA, Kyoichi NAKAJIMA, TECHNICAL REPORT OF IEICE AP2003-85, pp. 125-130, July 2003 “Simplified High Accuracy Measuring Method for Radio Equipment Using Integral Antennas, Radiated RF Power Measurement Using a Spherical Positioner (Part 2)”, Tomoyuki NOJIMA, Kyoichi NAKAJIMA and the electromagnetic wave coupling device of (4) in Japanese Patent No. 3436669.
A highly accurate measurement is possible by the spherical surface scanning method disclosed in TECHNICAL REPORT OF IEICE AP2002-61 (2002-7), pp. 29-34, July 2002 “Simplified High Accuracy Measuring Method for Radio Equipment Using Integral Antennas, Radiated RF Power Measurement Using a Spherical Positioner (Part 1)”, Tomoyuki NOJIMA, Kyoichi NAKAJIMA, and TECHNICAL REPORT OF IEICE AP2003-85, pp. 125-130, July 2003 “Simplified High Accuracy Measuring Method for Radio Equipment Using Integral Antennas, Radiated RF Power Measurement Using a Spherical Positioner (Part 2)”, Tomoyuki NOJIMA, Kyoichi NAKAJIMA. On the other hand, large equipment including a radio wave non-reflection chamber and a spherical surface scanner is required, and it takes a long time to measure.
Further, according to the spherical surface scanning method, radio waves are radiated to only a small part of the whole space and the power is determined from the total sum thereof. Therefore, the receiving sensitivity at each measurement point is so small that the problem is posed that the measurement of spurious radiation is difficult. In a UWB device, for example, the continuous spurious radiation is defined as −90 dBm/MHz and the impulsive spurious radiation as −84 dBm/MHz. It is very difficult to measure these spurious radiations by the measurement method described above.
The method in which the radio wave is agitated in a room covered with a metal, on the other hand, has the advantage that a large radio wave non-reflection chamber is not required. The problems of this method, however, are that the coincidence between the artificially generated random field and the theoretical probability model remains ambiguous, the statistical process on which the method is based makes a large inaccurate result, the measurement requires a long time, and the like. Another problem of this method is that, like in the spherical surface scanning method, the spurious radiation cannot be measured easily.
Also, the method using the G-TEM cell poses the problem that not only it is difficult to secure the uniformity of the internal field distribution but also the measurement of the total radiated power makes it necessary to arrange a two-axis rotary table in the G-TEM cell to make it possible to move the DUT in all directions.
Further, the method described in Japanese Patent No. 436669 requires a plurality of antennas, an isolator connected to each of the antennas, a phase regulator, a synthesizer configured to synthesize the signals of the array antennas, and the like. This poses, therefore, the problem that not only the system is complicated and high in cost but also the DUT is limited to the dipole antenna. Also, the measurement of the spurious radiation is difficult as in each of the aforementioned methods.
As a technique for solving these problems, the present inventors have already proposed a method to measure the total radiated power of the antenna using a spheroidal coupler disclosed in IEICE Technical Report AP2007-192 (2008-03), pp. 113-118 “Total radiated power (TRP) measurement of small radio terminals using a spheroidal coupler”.
In the method disclosed in this document, an enclosed space surrounded by an ellipsoidal metal wall surface obtained by rotating an ellipse around an axis connecting the focal points of the ellipse is formed, and at the focal points of the enclosed space of the ellipsoid, a DUT and a receiving antenna are arranged, respectively, so that the radio wave radiated from the DUT is reflected on the wall surface and concentrated at the receiving antenna thereby to measure the total radiated power of the DUT.
The method disclosed in the IEICE Technical Report AP2007-192 (2008-03), pp. 113-118 “Total radiated power (TRP) measurement of small radio terminals using a spheroidal coupler” is based on the principle that the radio waves output in various directions from a position in the neighborhood of the first focal point are reflected on the wall surface and concentrated in the neighborhood of the second focal point substantially at the same time. According to this method, only the primary reflection wave is extracted to measure the total radiated power in order to remove the effect of what is called the multiple reflection is avoided in which the radio wave passed through the neighborhood of the second focal point is reflected again on the wall surface and returned to the neighborhood of the first focal point, and after being reflected again on the wall surface, returns to the second focal point.
According to this method, the total radiated power can be measured without any problem as long as the DUT is small and the radiation characteristic thereof has no directivity. It has been found, however, in the case where the DUT is large in size and the beam radiated from the DUT is divided into a plurality of components or the radio wave output from the DUT has a side lobe, a cancellation phenomenon occurs in which the beams of different phases are concentrated in the neighborhood of the focal points to weaken the radio wave, thereby making it difficult to measure the total radiated power accurately.